In the development of a new therapy for brain pathologies, the blood-brain barrier (BBB) is considered as a major obstacle for the potential use of drugs for treating disorders of the central nervous system (CNS). The global market for CNS drugs was $33 billion in 1998, which was roughly half that of global market for cardiovascular drugs, even though in the United States, nearly twice as many people suffer from CNS disorders as from cardiovascular diseases. The reason for this lopsidedness is that more than 98% of all potential CNS drugs do not cross the blood-brain barrier. In addition, more than 99% of worldwide CNS drug development is devoted solely to CNS drug discovery, and less than 1% is directed to CNS drug delivery. This ratio could explain why no efficient treatment is currently available for the major neurological diseases such as brain tumors, Alzheimer's and stroke.
The brain is shielded against potentially toxic substances by the presence of two barrier systems: the blood-brain barrier (BBB) and the blood-cerebrospinal fluid barrier (BCSFB). The BBB is considered to be the major route for the uptake of serum ligands since its surface area is approximately 5000-fold greater than that of BCSFB. The brain endothelium, which constitutes the BBB, represents the major obstacle for the use of potential drugs against many disorders of the CNS. As a general rule, only lipophilic molecules smaller than about 500 Daltons may pass across the BBB, i.e., from blood to brain. However, the size of many drugs that show promising results in animal studies for treating CNS disorders is considerably bigger. Thus, peptide and protein therapeutics are generally excluded from transport from blood to brain, owing to the negligible permeability of the brain capillary endothelial wall to these drugs. Brain capillary endothelial cells (BCECs) are closely sealed by tight junctions, possess few fenestrae and few endocytic vesicles as compared to capillaries of other organs. BCECs are surrounded by extracellular matrix, astrocytes, pericytes and microglial cells. The close association of endothelial cells with the astrocyte foot processes and the basement membrane of capillaries are important for the development and maintenance of the BBB properties that permit tight control of blood-brain exchange.
International publication WO2004/060403 discloses an invention made by the inventors relating to molecules for transporting a drug across the blood brain barrier. Otherwise, to date, there is no efficient drug delivery approach available for the brain. Methods under investigation for peptide and protein drug delivery to the brain may be divided in three principal strategies. Firstly, invasive procedures include the direct intraventricular administration of drugs by means of surgery, and the temporary disruption of the BBB via intracarotid infusion of hyperosmolar solutions. Secondly, the pharmacologically-based strategy consists in facilitating the passage through the BBB by increasing the lipid solubility of peptides or proteins. Thirdly, physiologic-based strategies exploit the various carrier mechanisms at the BBB, which have been characterized in the recent years. In this approach, drugs are attached to a protein vector that performs like receptors-targeted delivery vehicle on the BBB. This approach is highly specific and presents high efficacy with an extreme flexibility for clinical indications with unlimited targets. The latter approach has been, and is still, investigated by the inventors, who came up with the molecules described in the afore-mentioned publication and those of the present invention.
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It would be highly desirable to be provided with improved molecules that can act as carriers or vectors for transporting a compound or drug across the BBB of an individual.